This invention relates to a method of growing on a substrate a semiconductor layer which is different in its constituent element or elements from the substrate to form a heterojunction therebetween.
Semiconductor techniques have recently made great strides and an extensive study and development have been made on "element" semiconductors such as silicon (Si) and germanium (Ge) and on various "compound" semiconductor such as gallium phosphide (GaP) and gallium arsenide (GaAs). Some are put to practical use. Such marked advance of the semiconductor techniques owes much to a method for preparing a semiconductor material. One of the most important problems in the study and developments of a method for preparation of a semiconductor material is to provide a method for forming on a substrate a semiconductor layer at least partially different in its constituent element or elements from the substrate, i.e. a method for forming a "semiconductor heterojunction" in a broad sense of the word. The "semiconductor heterojunction" herein defined is intended to include (1) a junction between different semiconductors such as a junction between different element semiconductors, a junction between an element semiconductor and a compound semiconductor or a junction between compound semiconductors at least partially different in its constituent element or elements from each other, and (2) a junction between a semiconductor and a material other than the semiconductor, such as a junction between a sapphire substrate and a semiconductor layer on the sapphire substrate. Formation of a good-quality semiconductor heterojunction theoretically possesses the way for an improvement on the characteristics of electronic or optical devices, or for realization of an entirely novel device. At the present time, merely a semiconductor laser device using a GaAs-GaAlAs heterojunction and an SOS structure (a single-crystal silicon layer is formed on a sapphire substrate) come into practical use. The reason for this is that it is generally difficult to form a semiconductor heretojunction, and that a desired semiconductor layer is not grown on a substrate different in its constituent element from the semiconductor layer or, even if a semiconductor layer is formed on such a substrate, it is often of a polycrystalline or amorphous type and not good in its crystallinity as a single crystal. It is generally said that a semiconductor heterojunction is more easily formed when it is formed between the materials which are close in their lattice constant and thermal expansion coefficient to each other. When, for example, gallium arsenide layer is formed on the germanium substrate, a relatively desirable state is obtained, but no expected germanium-gallium arsenide heterojunction has been formed up to this date and it is in a very fundamental stage from the standpoint of crystallinity.
A technique for forming a compound semiconductor on a silicon substrate is most expected to be realized in an attempt to obtain a semiconductor heterojunction. The reason is because light-emitting devices or light receiving devices can be formed on a silicon substrate of which the manufacturing technique is most advanced and which is inexpensive and wide in its application range. Moreover, much can be expected from the standpoint of costs as compared with the case where these devices are all made of a compound semiconductor, and these devices can be interpolated into an integrated circuit which utilizes a widely used silicon substrate. There is also a possibility that a heterojunction emitter transistor will be realized. The heterojunction emitter transistor is of a three-layer structure having, for example, a gallium phosphide layer on a substrate having a pn junction in it, for example, an n-p-n structure (i.e. an n-type gallium phosphide (emitter)-p-type silicon (base)-n-type silicon (collector) structure) and a p-n-p structure (i.e. a p-type gallium phosphide (emitter)-n-type silicon (base)-p-type silicon (collector) structure). For example, when a forward bias is applied between the base and the emitter of such an n-p-n transistor structure, electrons are injected from the emitter to the base of the transistor, but holes are not injected from the base to the emitter of the transistor because the width of forbidden band of the gallium phosphide is greater than that of the silicon. For this reason, the emitter injection efficiency is 100% and an improvement on the characteristics of the transistor is thus expected. The principle of a heterojunction emitter transistor has already been proposed in Proc. IRE, vol. 45,1957. Much has been expected for realization of a heterojunction between silicon and a compound semiconductor, but there has hardly been any report showing that a single crystalline compound semiconductor structure is formed on a single crystal silicon substrate having a relatively wide area of, for example, about 5 mm.times.5 mm. A report has been made by H. B. Pogge, B. M. Kemlage and R. W. Broadie in the U.S.A. in 1977, showing that a relatively-good-quality gallium phosphide layer is formed in two steps on a silicon substrate (see Journal of Crystal Growth, vol. 37, 1977, pp. 13 to 22). This method comprises growing a gallium phosphide layer on a silicon substrate through thermal decomposition of an organic compound of gallium using the chemical reaction with phosphine (PH.sub.3) and then growing a gallium phosphide layer (a second layer) through a Ga-PCl.sub.3 -H.sub.2 reaction. However, this method has the following disadvantages. Firstly, the method involves complicated steps and, in spite of a supposition that formation of gallium phosphide can be attained using either one of a gallium organic material-PH.sub.3 -H.sub.2 system and Ga-PCl.sub.3 -H.sub.2 system, it is necessary to use both the systems. This is very disadvantageous from the standpoint of a work process as well as of a device. Secondly, it is necessary to use harmful, high-explosive phosphine (PH.sub.3). PH.sub.3 is commercially available usually in such a state that it is contained at a lower concentration level in normal H.sub.2 gas. Even such a lower concentration of PH.sub.3 is very noxious and high-explosive. In spite of this fact, a fairly high concentration of PH.sub.3 is required in the step of the above-mentioned method, thus involving high risks during the process. Thirdly, a gallium phosphide layer obtained is poorer in its electrical properties. A gallium phosphide layer to be initially formed by a thermal decomposition method on the silicon substrate is formed not in such a sustantially equilibrium state as when a normally good-quality single crystal is grown, but under the condition that gallium phosphide produced through thermal decomposition is "deposited" onto the silicon substrate (in this case, gallium phosphide is deposited unconditionally onto the silicon substrate). For this reason, a boundary structure between the silicon substrate and the gallium phosphide layer (first layer) is disturbed and is undesirable from the standpoint of, for example, crystallographical and electrical properties. A first undesirable gallium phosphide layer, after a second gallium phosphide layer is deposited thereon, remains deposited as it is and an unnecessary junction can be formed between the first gallium phosphide layer and the second gallium phosphide layer. Though this junction is one between the semiconductors each having the same constituents, if such unnecessary junction is included there, the characteristics of, for example, electronic devices in general suffer disadvantages. Even in this sense, it can not be said that a silicon-gallium phosphide heterojunction is good.